The aims of the present study were to determine the free radical scavenging activity and antimicrobial activity against 15 food spoilage and pathogenic microorganisms of 18 selected Thai medicinal plants from 12 different families based on ethnopharmacological and taxonomic information. The methanolic extract of Allium sativum L. gave highest yield (34.76% db) following Rheum officinale Baill. (24.94% db) and Curcuma longa L. (23.39% db), respectively. Sixteen plant materials out of 18 selected Thai medicinal plants showed high effective scavenging capacity (IC50<0.06 mg mL-1) that order: Mansonia gagei Drumm., Pterocarpus santalinus L., Cinnamomum iners Blume., Kaempferia parviflora Wall., Caesalpinia sappan L., Rheum officinale Baill. and Albizia myriophylla Benth., respectively. The highest antioxidant activity was detected in Kaempferia parviflora Wall. by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. The most active medicinal plant extracts showed broad spectrum antimicrobial activity against 15 food spoilage and pathogenic microorganisms. The Allium sativum L., Caesalpinia sappan L., Mansonia gagei Drumm., Alpinia galangal Swartz., Curcuma longa L. and Kaempferia parviflora Wall. extracts had the greatest inhibitory effect against B. cereus ATCC 11778, E. coli ATCC 29214, Staph. aureus ATCC 13150, Pr. vulgaris TISTR 100, Strep. cremoris TISTR 058, Salm. typhi ATCC 43579 and C. krusei TISTR 5256 by using the disc diffusion method. The findings support the view that some Thai medicinal plants are promising sources of potential antioxidants and antimicrobial activity, which may be efficient as preventive agents in the pathogenesis of some diseases.